1. Field of the Invention
This invention relates to a prosthetic device that is used in the surgical reconstruction of a portion of the small bones that form the ossicular chain in the middle ear. More specifically, a prosthesis has been developed for partially rebuilding the incus when either all or part of the long process or lenticular process of the incus has eroded.
2. Description of the Related Art
Sound waves enter the outer ear canal and cause the tympanic membrane to vibrate at a rate which corresponds to the frequency of the sound waves. The vibrations are transmitted to the middle ear through a series of three tiny bones that form what is called the ossicular chain. These bones are the malleus, incus and the stapes. They operate to transport the vibrations from the tympanic membrane to the cochlea, enabling the hearer to perceive sound.
These bones are connected so the vibrations can be transmitted along the entire chain. The three bones are connected in series through joints that allow the bones to move relative to each other and transmit vibrations through a highly efficient lever action. The malleus is vibrated through its connection with the tympanic membrane. The malleus is connected through the incudo-malleolar joint to the incus, which enables vibrations to be imparted to the incus, through the incudo-stapedial joint and then to the stapes. The incudo-malleolar joint is significant in the faithful transmission of vibrations through the ossicular chain because of the relatively large contact area between the malleus and incus.
One of the most common maladies in the ossicular chain results from a lesion caused by chronic otitis media. This lesion can form on the lenticular process of the incus and causes the lenticular process and the connection between the incus and stapes to deteriorate. However, it can also form along the long process, leaving the incudo-stapedial joint intact.
Medical implants have been developed to reconstruct the ossicular chain when a portion of the incus is missing. However, all of the known implants involve complete replacement of the incus, which destroys the natural joint between the malleus and the incus as well as the joint between the incus and stapes. The resulting structure is less efficient than the natural joints because the implant replaces them with rigid connections.
Techniques have also been developed which use autograph cartilage from the host's body as a wedge between the remnant incus and stapes in order to reconstruct the ossicular chain. This technique also affects the natural action of the incudo-malleolar joint because the wedge is connected to the malleus and incus as well as the stapes. A combined medical prosthesis and magnet is suggested in U.S. Pat. No. 4,606,329 to Hough, which is implanted, among other places, between the incus and stapes. However, the device bypasses the normal vibratory function of the bones in the ossicular chain and imparts vibrations directly to the stapes.
In current practice, when the lenticular process of the incus has eroded, but the stapes and the main incus body are generally intact, the incus is surgically removed and entirely replaced with an implant. This practice requires the incudo-malleolar joint to be replaced with a rigid connection that does not effectively transmit the vibratory motion. It would a better practice to utilize the remaining natural bone of the incus without replacing it entirely with a prosthesis, thus leaving the incudo-malleolar joint intact.
Therefore, there is a clinical need for a prosthesis that allows reconstruction of a portion of the incus without complete replacement and connects to either the generally healthy stapes or the healthy portion of the lenticular process for maintaining most of the natural action of the ossicular chain.